The invention relates generally to apparatus for determining and indicating the position of an edge of a web that is to be acted on by a machine. In particular, the invention relates to the sensing of an edge of a metal web feeding a die-cutter which cuts the web into sheets to be used in making metal cans. It has been found that when a web edge is wrinkled or otherwise distorted the web can jump out of the roller guides that are used to position the web before it is cut. According to the present invention, variations in edge configuration over a predetermined amount are visually indicated to the operator of the die-cutting machine and/or the machine is automatically stopped so as to prevent damage thereto in the event the web does jump out from the roller guides.
According to the present invention, a physical sensing means having a tip of hard material is biased into engagement with an edge of a web feeding a machine for acting on the web. The physical sensing means is operatively connected to a linear variable differential transformer (LVDT) in a bracket specially adapted for holding the sensing means and the LVDT. The bracket ensures that no side forces will be exerted upon the LVDT, but rather only the linear translation of the sensing means will have an effect thereon. The LVDT generates an electrical signal in response to the relative position of the sensing means, and the electrical signal varies in magnitude in dependence upon the position of the sensing means relative to a fixed point on the machine, the bracket being operatively connected to the machine. Adjustment of the position of the sensing means is provided by the bracket.
Visual indication of the web's position is provided by three indicating lights, an "O.K." light indicating that the web is properly positioned, a "short" light indicating that the web edge is too far from the roller guides, and a "long" light indicating that the web edge is too close to the roller guides. The visual indication provided by these lights is initiated by circuitry connected to the LVDT. The d.c. signal generated by the LVDT is compared to a reference voltage by differential comparators. These comparators are connected in series to switching amplifiers (transistors) which drive a triple exclusive OR gate. Each gate of the triple exclusive OR gate is in turn connected in series with a transistor that drives one of the three indicating lamps, and optionally also drives a light emitting diode corresponding to the indicating lamp, and/or other indicating or shut-off means. Potentiometers connected between the differential comparators and the LVDT provide for adjustment of the relative positions of the web edge at which the lights will be lit. In this way, only one light can be lit at a time and such lights are lit only when the edge of the web has a particular position relative to the reference point on the machine.
While in the past there have been many proposals for sensing various conditions of webs being fed to machines, none have provided positive means for allowing physical adjustment and only linear movement of sensing means associated with a LVDT, nor for providing visual indication of which of three positions a web is in with respect to a reference point on the machine, nor the particular means and method according to the invention. For instance, U.S. Pat. No. 3,100,889 shows a pair of LVDTs responsive to opposite edges of a web for indicating the width of the web. While such means are generally useful, they do not sense the position of the web with respect to a fixed point on the machine being fed, do not provide means for eliminating side forces on the LVDTs, and require two LVDTs instead of one. Also, since a.c. is used therewith, integrating means must be employed, making the device relatively complicated as compared to the present invention. U.S. Pat. No. 2,651,031 shows the position of edge portions of a web with respect to fixed portions of a machine, however does not provide for visual indications of the web in three positions thereof, and requires that the web be conductive. U.S. Pat. Nos. 3,278,838, 3,147,574, 3,519,922 and 2,896,198 provide various other means for sensing various physical properties of a web, however none of these means have the particular features or advantageous results according to the method and apparatus of the present invention.
It is the primary object of the present invention to provide a simple effective apparatus for positively sensing and indicating the relative position of an edge of a web being fed to a machine for acting thereon. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.